The invention refers to a device for measuring the capacitance electrical wires.
In the manufacture of electrical wires comprising a conductor and an insulating cover or sheathing it is important for the manufacturer to know the capacitance of the wire or cable. As known, the capacitance influences the resistance impedance of the cable.
From GB 2 003 613 it has become known to measure the capacitance of a cable in that a measuring tube of electrically conductive material is provided through which the cable is extend. Preferably, the measuring tube is located in a cooling path and thus is filled with water. A measuring voltage of high frequency is applied to the measuring tube while the conductor is connected to ground. Thereby, a current flows between the measuring tube and the conductor, the current depending upon the dielectric constant of the insulating material and the thickness thereof.
From EP 0 679 863 it has become know to use the mentioned measuring principle to measure the eccentric displacement of the color of a cable within the cover. To this purpose three arcuate electrode segments are arranged about the circumference of the cable. The segments are circumferentially spaced and electrically insulated relative to each other. Each segment or electrode is supplied with a measure voltage of high frequency. The current flowing to the individual electrode segments is a measure-for the thickness of the cover in the area of the electrode segments. In case the currents flowing to the individual electrode segments are different, this is an indication the conductor is not concentrically arranged in the cable cover.
In particular with electrical wires operated with high frequency, e.g. 2 GI-Iz also local changes of the capacitance along the wire may be important. Such change""s an be caused by an inhomogeneous structure of the cable or by changes of the thickness. Such changes cause undesired reflections of electromagnetic waves.
It is an object of he invention to provide a device for measuring the capacitance of electrical wires which allows also the measurement of local changes of the capacitance.
In the device according to the invention a second measuring tube is co axially arranged with respect to a first measuring tube. The length of the second tube is significantly smaller than that of the first tube. The first tube may be comprised of a plurality of individual tube portions. The first tube has to have a minimum length if a precise capacitance measurement is desired. Such minimum length is for example 200 mm. Such a length, however, does not allow a satisfactory resolution in order to determine changes of the capacitance in smaller areas. Therefore, a second significantly short measuring tube is provided which has a length which is for example 10 to 25% of the total length of the first and second measuring tube. Thus, a second tube may have a length between 20 and 50 mm. By means of suitable capacitance measuring means the capacitance between the first measuring tube and the conductor and the capacitance between the second measuring tube and the conductor are measured. It is also possible, in view of the first capacitance values to take the fit and second measuring tubes together and to measure the capacitance between these tubes and the conductor.
The capacitance can be detained in a known manner by the measurement of current or voltage. For example the first and second measuring tube may be connected to separate current measuring devices, and a suitable evaluation device evaluates changes of the current to indicate them as changes of the capacitance. In case the changes reach critical values a corresponding indication can be carried out. Occasionally, a portion of the cable has to be eliminated
Another possibility for measuring the capacitance can be carried out by a bridge circuit in that the voltage drop of a known capacitor is measured which is series-connected to the capacitance to be measured, with the circuit being supplied with a known voltage.
In the general measuring of the capacitance the second measuring tube may be excluded. In an alternative case, it is included in that the currents flowing to the first and the second tube are added. In this case a current measuring device may measure the total current flowing to both measuring tubes. The inclusion of the second measuring has the advantage that the total length of a device for measuring the capacitance must not be larger than that of conventional measuring devices.
It is understood that also two or more than two measuring tubes may be provided which occasionally may have different length in order to achieve the desired degree of resolution. The local resolution increases with a decrease of the length of the measuring tube, however, the disturbing effects also increase with decreasing length so that the danger exist that with particular short second tubes no usable measuring results can be achieved.
In a particular embodiment of the invention the second measuring tube is located between at least two portions of the first measuring tube. Such an arrangement has the advantage that the disturbing fields effecting on the second measuring tubs are minimal so that changes of the capacitance can be particularly precisely measured.
Preferably, the second measuring tube has the same diameter as the first measuring tube, and both measuring tubes are preferably surround by a common screening tube. An insulation of a solid material can be provided between the screening tube and the measuring tubes which interconnects both parts forming an integral body. Additionally, a second screening tube connected to ground may surround the"" first screening tube and forms a unit with the other parts of the device.